Fair Value Measurement - Financial Instruments Carried at Fair Value (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Assets:
Fixed-maturity securities	$ 11,136	$ 11,301
Other invested assets	102	94
Liabilities:
Credit derivative liabilities	258	271
Fair Value Measured at Net Asset Value Per Share [Member]
Liabilities:
Investment funds, fair value	41	45
Recurring [Member]
Assets:
Other invested assets	7	7
FG VIEs’ assets, at fair value	627	700
Other assets	124	123
Total assets carried at fair value	11,894	12,131
Liabilities:
Credit derivative liabilities	258	271
FG VIEs’ liabilities with recourse, at fair value	571	627
FG VIEs’ liabilities without recourse, at fair value	108	130
Total liabilities carried at fair value	937	1,028
Recurring [Member] \| Level 1 [Member]
Assets:
Other invested assets	0	0
FG VIEs’ assets, at fair value	0	0
Other assets	25	25
Total assets carried at fair value	574	489
Liabilities:
Credit derivative liabilities	0	0
FG VIEs’ liabilities with recourse, at fair value	0	0
FG VIEs’ liabilities without recourse, at fair value	0	0
Total liabilities carried at fair value	0	0
Recurring [Member] \| Level 2 [Member]
Assets:
Other invested assets	0	0
FG VIEs’ assets, at fair value	0	0
Other assets	40	36
Total assets carried at fair value	9,264	9,608
Liabilities:
Credit derivative liabilities	0	0
FG VIEs’ liabilities with recourse, at fair value	0	0
FG VIEs’ liabilities without recourse, at fair value	0	0
Total liabilities carried at fair value	0	0
Recurring [Member] \| Level 3 [Member]
Assets:
Other invested assets	7	7
FG VIEs’ assets, at fair value	627	700
Other assets	59	62
Total assets carried at fair value	2,056	2,034
Liabilities:
Credit derivative liabilities	258	271
FG VIEs’ liabilities with recourse, at fair value	571	627
FG VIEs’ liabilities without recourse, at fair value	108	130
Total liabilities carried at fair value	937	1,028
Fixed Maturities [Member]
Assets:
Fixed-maturity securities	10,225	10,674
Fixed Maturities [Member] \| Obligations of state and political subdivisions [Member]
Assets:
Fixed-maturity securities	5,289	5,760
Fixed Maturities [Member] \| US government and agencies [Member]
Assets:
Fixed-maturity securities	283	285
Fixed Maturities [Member] \| Corporate securities [Member]
Assets:
Fixed-maturity securities	1,983	2,018
Fixed Maturities [Member] \| RMBS [Member]
Assets:
Fixed-maturity securities	844	861
Fixed Maturities [Member] \| CMBS [Member]
Assets:
Fixed-maturity securities	546	549
Fixed Maturities [Member] \| Asset-backed Securities [Member]
Assets:
Fixed-maturity securities	968	896
Fixed Maturities [Member] \| Foreign government securities [Member]
Assets:
Fixed-maturity securities	312	305
Fixed Maturities [Member] \| Recurring [Member]
Assets:
Fixed-maturity securities	10,225	10,674
Fixed Maturities [Member] \| Recurring [Member] \| Obligations of state and political subdivisions [Member]
Assets:
Fixed-maturity securities	5,289	5,760
Fixed Maturities [Member] \| Recurring [Member] \| US government and agencies [Member]
Assets:
Fixed-maturity securities	283	285
Fixed Maturities [Member] \| Recurring [Member] \| Corporate securities [Member]
Assets:
Fixed-maturity securities	1,983	2,018
Fixed Maturities [Member] \| Recurring [Member] \| RMBS [Member]
Assets:
Fixed-maturity securities	844	861
Fixed Maturities [Member] \| Recurring [Member] \| CMBS [Member]
Assets:
Fixed-maturity securities	546	549
Fixed Maturities [Member] \| Recurring [Member] \| Asset-backed Securities [Member]
Assets:
Fixed-maturity securities	968	896
Fixed Maturities [Member] \| Recurring [Member] \| Foreign government securities [Member]
Assets:
Fixed-maturity securities	312	305
Fixed Maturities [Member] \| Recurring [Member] \| Level 1 [Member]
Assets:
Fixed-maturity securities	0	0
Fixed Maturities [Member] \| Recurring [Member] \| Level 1 [Member] \| Obligations of state and political subdivisions [Member]
Assets:
Fixed-maturity securities	0	0
Fixed Maturities [Member] \| Recurring [Member] \| Level 1 [Member] \| US government and agencies [Member]
Assets:
Fixed-maturity securities	0	0
Fixed Maturities [Member] \| Recurring [Member] \| Level 1 [Member] \| Corporate securities [Member]
Assets:
Fixed-maturity securities	0	0
Fixed Maturities [Member] \| Recurring [Member] \| Level 1 [Member] \| RMBS [Member]
Assets:
Fixed-maturity securities	0	0
Fixed Maturities [Member] \| Recurring [Member] \| Level 1 [Member] \| CMBS [Member]
Assets:
Fixed-maturity securities	0	0
Fixed Maturities [Member] \| Recurring [Member] \| Level 1 [Member] \| Asset-backed Securities [Member]
Assets:
Fixed-maturity securities	0	0
Fixed Maturities [Member] \| Recurring [Member] \| Level 1 [Member] \| Foreign government securities [Member]
Assets:
Fixed-maturity securities	0	0
Fixed Maturities [Member] \| Recurring [Member] \| Level 2 [Member]
Assets:
Fixed-maturity securities	8,862	9,410
Fixed Maturities [Member] \| Recurring [Member] \| Level 2 [Member] \| Obligations of state and political subdivisions [Member]
Assets:
Fixed-maturity securities	5,197	5,684
Fixed Maturities [Member] \| Recurring [Member] \| Level 2 [Member] \| US government and agencies [Member]
Assets:
Fixed-maturity securities	283	285
Fixed Maturities [Member] \| Recurring [Member] \| Level 2 [Member] \| Corporate securities [Member]
Assets:
Fixed-maturity securities	1,920	1,951
Fixed Maturities [Member] \| Recurring [Member] \| Level 2 [Member] \| RMBS [Member]
Assets:
Fixed-maturity securities	533	527
Fixed Maturities [Member] \| Recurring [Member] \| Level 2 [Member] \| CMBS [Member]
Assets:
Fixed-maturity securities	546	549
Fixed Maturities [Member] \| Recurring [Member] \| Level 2 [Member] \| Asset-backed Securities [Member]
Assets:
Fixed-maturity securities	71	109
Fixed Maturities [Member] \| Recurring [Member] \| Level 2 [Member] \| Foreign government securities [Member]
Assets:
Fixed-maturity securities	312	305
Fixed Maturities [Member] \| Recurring [Member] \| Level 3 [Member]
Assets:
Fixed-maturity securities	1,363	1,264
Fixed Maturities [Member] \| Recurring [Member] \| Level 3 [Member] \| Obligations of state and political subdivisions [Member]
Assets:
Fixed-maturity securities	92	76
Fixed Maturities [Member] \| Recurring [Member] \| Level 3 [Member] \| US government and agencies [Member]
Assets:
Fixed-maturity securities	0	0
Fixed Maturities [Member] \| Recurring [Member] \| Level 3 [Member] \| Corporate securities [Member]
Assets:
Fixed-maturity securities	63	67
Fixed Maturities [Member] \| Recurring [Member] \| Level 3 [Member] \| RMBS [Member]
Assets:
Fixed-maturity securities	311	334
Fixed Maturities [Member] \| Recurring [Member] \| Level 3 [Member] \| CMBS [Member]
Assets:
Fixed-maturity securities	0	0
Fixed Maturities [Member] \| Recurring [Member] \| Level 3 [Member] \| Asset-backed Securities [Member]
Assets:
Fixed-maturity securities	897	787
Fixed Maturities [Member] \| Recurring [Member] \| Level 3 [Member] \| Foreign government securities [Member]
Assets:
Fixed-maturity securities	0	0
Short-term Investments [Member]
Assets:
Fixed-maturity securities	911	627
Short-term Investments [Member] \| Recurring [Member]
Assets:
Fixed-maturity securities	911	627
Short-term Investments [Member] \| Recurring [Member] \| Level 1 [Member]
Assets:
Fixed-maturity securities	549	464
Short-term Investments [Member] \| Recurring [Member] \| Level 2 [Member]
Assets:
Fixed-maturity securities	362	162
Short-term Investments [Member] \| Recurring [Member] \| Level 3 [Member]
Assets:
Fixed-maturity securities	$ 0	$ 1

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